The books themselves aren't canon to the t.v series, the book actually having a number of inconsistencies when compared to the show not helping it. But even if Cadence was once a pegasus the flashback showed her as an alicorn when foalsitting Twilight. Sorry I hoe this didn't seem rude.

You ever wonder why a Princess was being a foalsitter? I mean, did Cadence need extra pizza money?

I think that the Alicorns have had something planned for Twilight all along. She's like the Chosen One of Equestria and they've been guarding/training her from the beginning. Remember what Cadence said to her back then?

Young Twilight Sparkle: I am so lucky to have you as my foalsitter!Young Cadance: I'm the one who's lucky, Twilight.Young Twilight Sparkle: [scoffs] You're a princess. I'm just a regular old unicorn.Young Cadance: You are anything but a regular old unicorn.

Was she just making a little unicorn feel better? Or did she let something slip?

Foalsitter or babysitter, I know if she was a babysitter (herself as a alicorn, not humanized) she would be the best. What kid wouldn't wish to have such a awesome sitter like her? I'm dreaming but... damn! She's... just so much lovable.